Inexpensive cameras based on CMOS imaging arrays are now found in a number of devices such as cellular telephones and PDAs. These cameras provide automatic exposure control based on the image received by the imaging array itself. These cameras utilize an electronic shutter to capture the image. The exposure time is determined by continually reading out the image and adjusting the exposure time such that the image, on average, is neither overexposed nor underexposed. Each pixel in the imaging array has a linear range in which the pixel generates a charge that is proportional to the light received during the exposure time. If the pixel is overexposed, the excess charge is lost, and hence, the pixel output remains at some maximum value independent of the light exposure.
Analogous problems occur if the light levels are too low. During the image readout, the charge in each pixel is readout and converted to a digital value. The accuracy of the analog-to-digital converter (ADC) and noise levels in the pixel set the lower limit on image intensity that can be accurately captured. If the light levels are less than the levels corresponding to one count in the output of the ADC, all of the light levels will be assigned the same numerical value.
The goal of the exposure control system is to maintain the exposure time in a range that assures that most of the pixels have outputs in the linear region. In one type of prior art system, a frequency distribution of the intensity values from the various pixels in the imaging array is generated for each exposure time. If the image is overexposed, the distribution will be concentrated at or near the maximum allowed intensity value. Similarly, if the image is underexposed, the distribution will be concentrated in the low pixel intensity values. This type of scheme requires that the entire image be readout and processed at each exposure time.
For imaging arrays having millions of pixels and inexpensive processors, the readout time can be too long to provide good exposure control. This is particularly true in situations in which the scene to be captured is changing rapidly. Live performances often involve scenes that are changing rapidly because the performers are moving around the stage and various lighting effects are actuated for short periods of time. By the time the processor in the camera determines the correct exposure time, the scene will have changed and that exposure time will no longer be valid. As a result, the cameras provided in cellular telephones and the like are of limited use in capturing scenes from such events.